[flang] Extension: don't require commas between most edit descriptors in formats

[klausler]

Summary

The standard explicitly allows a comma to be omitted between a 'P'
edit descriptor and a following numeric edit descriptor (e.g., 1PE10.1),
and before and after a '/' edit descriptor, but otherwise requires them
between edit descriptors. Most implementations, however, only require
commas where they prevent ambiguity, and accept things like 1XI10.
This extension is already assumed by the static FORMAT checker in
semantics. Patch the runtime to behave accordingly.

flang/docs/Extensions.md

 * Format expressions that have type but are not character and not
   integer scalars are accepted so long as they are simply contiguous.
   This legacy extension supports pre-Fortran'77 usage in which
   variables initialized in DATA statements with Hollerith literals
   as modifiable formats.
 * At runtime, `NAMELIST` input will skip over `NAMELIST` groups
   with other names, and will treat text before and between groups
   as if they were comment lines, even if not begun with `!`.
+* Commas are required in FORMAT statements and character variables
+  only when they prevent ambiguity.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/runtime/format-implementation.h

if (ch == '(') {
       context.Emit(format_ + offset_, static_cast<std::size_t>(*repeat));
       offset_ += *repeat;
     } else if (ch >= 'A' && ch <= 'Z') {
       int start{offset_ - 1};
       CharType next{'\0'};
       if (ch != 'P') { // 1PE5.2 - comma not required (C1302)
         CharType peek{Capitalize(PeekNext())};
         if (peek >= 'A' && peek <= 'Z') {
+          if (ch == 'A' /* anticipate F'202X AT editing */ || ch == 'B' ||
+              ch == 'D' || ch == 'E' || ch == 'R' || ch == 'S' || ch == 'T') {
+            // Assume a two-letter edit descriptor
             next = peek;
             ++offset_;
+          } else {
+            // extension: assume a comma between 'ch' and 'peek'
+          }
         }
       }
       if ((!next &&
               (ch == 'A' || ch == 'I' || ch == 'B' || ch == 'E' || ch == 'D' ||
                   ch == 'O' || ch == 'Z' || ch == 'F' || ch == 'G' ||
                   ch == 'L')) ||
           (ch == 'E' && (next == 'N' || next == 'S' || next == 'X')) ||
           (ch == 'D' && next == 'T')) {